Apex domains cannot have CNAME records.
Question and response
Question
dig pop-hannover.de. NS
Answer
pop-hannover.de. 5 NS ns.pop-hannover.de.
pop-hannover.de. 5 NS ns2.pop-hannover.net.
Authority
The response has no authority records.
Additional
ns.pop-hannover.de. 5 A 193.98.1.200
ns.pop-hannover.de. 5 AAAA 2a00:e68:0:0:0:0:0:2
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0